The feline immunodeficiency virus (FIV) animal model presents a unique and significant opportunity for assessing novel antiviral strategies for HIV/AIDS in humans. However, characterization of FIV accessory genes that share conserved functions with HIV-encoded proteins will be crucial to development of this animal model for characterizing targeted intervention strategies useful for treatment of human immunodeficiency virus-1 (HIV-1) infection of humans. FIV gene OH-A, also designated orf-2, encodes a 77 amino acid accessory protein. Our recent studies have shown Orf-A to be important in late steps of the FIV life cycle involved in virion formation and in early steps involved in virus infectivity and have mapped critical Orf-A domains needed for these steps in replication. In a separate study we reported novel previously unrecognized Orf-A functions. Expression of GFP-Orf-A fusion proteins in mammalian cells demonstrated nuclear localization for this FIV accessory protein and facilitated mapping of a nuclear localization signal (NLS) critical for nuclear import of FIV Orf-A. Lastly, assessment of cell cycle profiles of cells transiently expressing GFP-Orf-A demonstrated that Orf-A causes an arrest at the second gap (G2) of the cell cycle. Based on these novel findings, we hypothesize that FIV oH-A encodes a viral protein that expresses specific properties similar to those expressed by HIV-1 Vpr. Furthermore, we hypothesize that specific oH.A protein-protein interactions may be common properties of accessory genes encoded by either non-primate lentiviruses (FIV and caprine arthritis encephalitis virus) or primate lentiviruses (HIV and simian immunodeficiency virus). We propose to further characterize FIV Orf-A by investigating Orf-A functions in virus replication, Orf-A domains critical for replication, and mechanisms by which Off-A affects virus replication. Importantly, we will compare effects of RV Orf-A and HIV Vpr expression and subcellular localization on host cell functions including cell cycle and cell viability. Lastly, we will examine effects of Orf-A function on viral pathogenesis through experimental cat inoculation studies testing FIV molecular clones encoding Orf-A mutations. The FIV animal model will be used to analyze the impact of specific viral gene activities (G2 arrest, apoptosis, etc.) shared by FIV Off-A and HIV-1 Vpr on viral pathogenesis in vivo and to accordingly examine the value of specific Vpr domains as targets for antiviral therapeutics based on HIV-1 Vpr.